It can become necessary to search for the leading free end of yarn being wound in the production of cops in a ring spinning frame as well as during further processing of the cops in a bobbin winding machine. While in a ring spinning frame the yarn connection for continuing the spinning process is produced directly, it is the conventional practice in a bobbin winding machine to perform a so-called complete cop preparation. In the process, the located yarn end is placed on the cop in a manner such that it can be easily located and grasped again later at a winding station.
In a ring spinning frame, the yarn end rests against the cop cone after a yarn break, since the progressive placement of yarn windings always takes place directly on the cop cone and advances therefrom in the direction of the cop nose. Accordingly, with each yarn break the height of the cop cone varies as a function of the state of completion of winding of the cop.
In bobbin winding machines, the yarn end of cops coming directly from the spinning frame has usually been deposited as a so-called overwinding or underwinding in the area of the bobbin foot or the bobbin tip and must be located and grasped at that location. However, so-called residual cops (i.e. cops having a usable amount of yarn but being less than fully wound) are also ejected from the winding stations if a yarn break on the cop has occurred during unwinding of the yarn or if the yarn connection was not reestablished after a yarn break during spinning, such as is the case in connection with conventional spinning frames being produced today. As a rule, the yarn end of these residual cops is also located on the cop cone which, depending on the state of unwinding of the cop, can also be disposed at a different level. Finally, there is also the possibility that cops are delivered to the winding machine by the ring spinning frame which are not fully wound because of the occurrence of a batch change.
It is necessary in all of the aforementioned cases to position yarn releasing or yarn grasping elements so that their distance from the cop cone is always of the same dimension and so that a high degree of success is assured in the yarn search. Lifting mechanisms for the yarn grasping elements are provided for this purpose.
Sensors which detect the position of the cop cone must be provided for appropriately triggering the lifting mechanisms. Photo-optical sensors are known to serve this purpose and, together with the yarn grasping elements, are moved along the longitudinal axis of cops by the lifting device.
Sensors of this type operate on the basis of reflections by measuring the light reflected by the bobbin surface. When associated light beams enter the area of the cop cone, the light is clearly more scattered, which can be detected by means of an appropriately disposed photo-optical sensor (for example, see Japanese Patent Publication JP-AS 45 15 814). Disadvantageously, however, because of the displacement of the yarn layers on the bobbin, a clear detection of the start of the cop cone is no longer possible. In contrast, the detection of the position of the cop cone is improved if a photoelectric barrier spaced apart from the bobbin is utilized as the sensor, since it is directly disrupted by the cop cone. Such a solution is known for example from German Patent Publication DE 26 12 660 A1 and European Patent Publication EP 0 322 008 B1.
A photo-optical sensor system is both relatively expensive and furthermore not reliable over time, since soiling occurs on the surface of the optical element because of the dust typically created by the conventional textile operations, which can lead to errors in the detection of the cop cone.